Due to the continually increasing cost of fuel, such as natural gas, greater emphasis has been placed on fuel efficiency in the design and construction of heating apparatus, such as furnaces and water heaters. One development has been the replacement of the conventional pilot light with either direct spark ignition, intermittent pilot ignition or hot surface ignition. Another development has been the use of a helically wound tubular heat exchanger to circulate the products of combustion through a hot water storage tank, by which heat is transferred from the products of combustion to the water in the tank.
Combination water heating and space heating apparatus, as shown, for example, in U.S. Pat. Nos. 4,541,410, 4,641,631 and 4,766,883, are known in the art. In such combination apparatus, hot water is circulated between a hot water storage tank and an hydronic heat exchanger coil located in an air supply duct to heat the air passing over the heat exchanger coil, thereby providing heated air to an indoor space. One problem associated with such prior art combination apparatus is the allocation of hot water between space heating and the normal hot water supply (e.g., hot water for domestic use). If there is a concomitant demand for space heating and for domestic hot water (e.g., showers, laundry, dishwasher, etc.), the hot water supply may be insufficient to satisfy the demand. Another problem is that the combustion chamber of the water heater is typically located at the bottom of the tank. As such, water in the tank is in direct contact with the sides and top of the combustion chamber, but not the bottom, thereby detracting from the efficiency of the apparatus. Further, water heated at the bottom of the tank rises to the top of the tank and is replaced by colder water sinking to the bottom of the tank. This natural convection is advantageous in terms of circulating water throughout the tank, but is disadvantageous in terms of being able to rapidly heat water to a desired temperature for immediate use (e.g., for space heating).
There is, therefore, a need for an improved water heating apparatus and for an improved combination water heating and space heating apparatus.